Add bool type corresponding to the llvm i1/ C99 bool

The idea is to find a way to represent the llvm i1 type, since llvm
ExecutionEngine also uses libffi now. It's a single bit, but I thought
I'd try to coerce it from an i8 (char). This proof-of-concept requires
more work, especially in the area of tests.
`conser` here should be `immediater` since `Val_of_bool` doesn't allocate.